This invention is directed to the measurement of moisture content of earth formation in the vicinity of a well borehole using a borehole probe comprising a source of fast neutrons and at least one epithermal neutron detector. More particularly, the invention is directed to high spatial resolution moisture measurements applicable to laminated or thinly bedded formations penetrated by a borehole wherein the borehole is filled with gas, and to fracture or vug detection wherein the borehole is filled with liquid.
Probes containing neutron sources have been used for many years to measure the water content of geologic formations penetrated by a well borehole. The physical principles underlying this type of measurement are based upon the facts that (a) the concentration of the element hydrogen within a formation dominates the neutron diffusion process within the formation, and (b) most hydrogen found in the more common earth formations is in the form of water (H.sub.2 O). Therefore any measure of the neutron properties of earth formation can be directly related to the water content of the formation. In the petroleum and minerals industries, the measurements of the neutron diffusion properties of earth formations as a function of depth within a borehole penetrating the formations are referred to as neutron "logs". The mining, agriculture and civil engineering industries have also employed neutron diffusion related measurements to determine the moisture content of soil, both in boreholes and at the surface of the earth. Instruments used to perform these measurements have been traditional referred to as "moisture gauges" in these industries.
Accurate and precise measurements of water content in geologic material is obviously desirable. A high degree of spatial resolution in the water content measurement is also desirable, and even critical, in many applications. An engineering application might, as an example, require high vertical resolution in a borehole measurement to determine (a) if water is distributed homogeneously throughout an interval of formation, or (b) if the same average water content is spatially distributed in thin beds containing a relatively high water content laminated alternately with other thin, essentially dry beds. Accuracy and precision of the measurement is controlled by a number of factors including the strength of the neutron source, the efficiency of the neutron detectors, and the time intervals over which the measurements are made. Spatial resolution, however, is a strong function of the distance or "spacing" between the neutron source and the neutron detector. Spatial resolution is affected by other factors such as the actual water content of the formation, but the dominating factor is source to detector spacing. In general, decreasing the spacing increases the spatial resolution of the measurement. In theory, maximum resolution is obtained when the spacing between a point source and a point detector is zero. In practice, the neutron source and the neutron detector will both have finite dimensions, and spacing is defined as the distance between the center of the source and the center of the detector. Maximum spatial resolution is approached as the source and detector are positioned as close together as physically possible.
Now turning specifically to borehole measurements, vertical spatial resolution along the axis if the borehole (vertical resolution) is maximized when the neutron source and the neutron detector are placed on a common plane which is normal to the axis of the borehole. This is zero "effective" spacing in borehole logging geometry and yields the maximum vertical resolution of water content measurements as the borehole is logged. The definitions of "vertical resolution" have varied in the neutron logging industry as well as in the neutron moisture gauge industry. The definitions usually fall under one of two criteria. The first criterion is related to the thinnest bed or zone which can be measured with a logging instrument in which the response of the instrument reaches a set percentage of the response in a homogeneous formation. The second criterion is related to the impulse response function (normally taken as full-width-half-maximum) or a step function (10% to 90% of the change in the response) or some other parameter which relates to the presence or absence of a thin bed. Associated with both criteria is a definition associated with the Fourier transform of the input function or step function response or some section of continuous log. This involves the frequency response of the neutron detector itself as well as other factors such as system noise level. In any definition of vertical resolution, the specific conditions should be specified. Regardless of the definition used, however, vertical resolution tends to increase as the distance between the neutron source and the neutron detector decreases.
Zero spaced thermal neutron measurements in the form of neutron moisture gauges, which essentially are zero spaced neutron logs, both surface and downhole, have been used since about 1950 as reported in Neutron Moisture Gauges, Technical Report Series 112, (1970), International Atomic Energy Authority, Vienna. The vertical resolution of an early neutron moisture gauge with a source to detector spacing near 9 cm. has been reported by C. H. M. van Bavel et al, Vertical Resolution in the Neutron Method for Measuring Soil Moisture, Transactions of the American Geophysical Union, (1954)Vol. 35, pp. 595-600. These authors reported that vertical resolution is a function of the moisture content of the formation in addition to being a function of source to detector spacing. The works of J. W. Nyhan et al, Spatial Resolution of Soil Water Content by Three Neutron Moisture Gauges, Los Alamos National Laboratory (1983) Report No. LA-UR-83-2863 showed that as source to detector spacing decreased, the effects of moisture content of the formation on vertical resolution increases. There are numerous references in the literature describing neutron logging systems and moisture gauges employing one or more thermal neutron detectors at a non-zero source to detector spacing, one or more thermal neutron detectors at a zero source to detector spacing, and one or more epithermal neutron detectors at non-zero spacings. J. R. Hearst et al, A Comparison of the Moisture Gauge and the Neutron Log in Air-Filled Holes, Proceedings of the Fifth International Symposium on Geophysics, Paper O, Tulsa, Okla. Oct. 24-28, 1993provides an excellent review and source of references. No reference is made to any moisture gauge or neutron logging device employing the detection of epithermal neutrons at zero spacing. It should be noted that there are advantages and disadvantages in measuring neutrons in the thermal and epithermal range in moisture gauges, especially in borehole applications. Very briefly, for a given formation, source to detector spacing and fast neutron source, the flux of thermal neutrons in the region of the detector is normally greater than the flux of epithermal neutrons. When using equal volume thermal and epithermal neutron detectors, with the latter consisting of cadmium wrapped around a thermal neutron detector, the count rate using the thermal neutron detector is normally larger than that of the epithermal detector. The statistical error in moisture gauges is therefore normally less when detecting neutrons in the thermal range rather that in the epithermal range. However, epithermal neutrons are much less sensitive to changes in the formation or type of liquid contained within the formation, and in particular, are almost insensitive to any changes in salinity of the fluids contained in the surrounding media. Stated another way, the epithermal neutron log is much less dependent on changes in the thermal neutron cross section or presence of high thermal neutron cross section elements in the volume surrounding the probe than is a thermal neutron log.
It is known that sensitivity of shod spaced neutron moisture gauges for measuring moisture in formations is greatly reduced when the borehole is liquid filled rather than air filled. Therefore, in prior work, the long spaced neutron log has been used to measure moisture content of formations in liquid filled boreholes. It has been found that with the current invention that in liquid filled boreholes, the probe is quite sensitive to non-uniform formations such as those resulting from fractures or rugs or any form of liquid filled cavities in the formation close to a zero-spaced epithermal neutron probe. The current invention can therefore be used as a fracture or rug detector in liquid filled boreholes.
In light of the above discussion, it is apparent that a high resolution, neutron based moisture gauge designed to operate in gas filled boreholes would incorporate the feature of zero effective source-detector spacing to optimize vertical resolution, and also incorporate the detection of epithermal neutrons to obtain accurate and precise measurements in gas filled boreholes.